This invention relates to a ventilation system, particularly to a natural ventilation system for an enclosed space, such as, home, school or large commercial and industrial building which includes fresh air inlets and fresh air supply ducts at the lower side of the wall and exhaust air discharge duct and exhaust air inlet ports for drawing the warm air into the discharge ducts provided in the ceiling of the room. The hot electric appliances and the heat of the human bodies provide a motivative force to the room air to rise into the discharge ducts and the high density and cold fresh air is drawn into the room.